The present invention relates in general to transformers and in particular to a direct current (“DC”) voltage transformer that utilizes pulse modulation.
Referring to FIG. 1, a prior art DC voltage transformer includes a power switch 1, for example a field effect (CMOS) transistor, whose control input receives an unmodulated oscillating reference signal Uref. FIG. 2 illustrates a representative waveform of the reference signal. The main output terminals of the power switch 1, between which a current flow can be regulated by the reference signal, are connected in series with an inductance 2 between two power supply potentials Vcc and GND. A first smoothing capacitor 3 is connected directly between the two power supply potentials. A second smoothing capacitor 4 is arranged in series with a Zener diode 5, with the capacitor 4 and diode 5 being connected in parallel with the main terminals of the power switch 1. The capacitance of the smoothing capacitors 3, 4 is chosen as a function of the frequency of the reference signal and the inductance value 2 so that there is a sufficiently constant output voltage on the output terminal Vout of the transformer, which is taken from a location between the second smoothing capacitor 4 and the Zener diode 5. The transformer output voltage is generally proportional to the pulse duty factor of the reference signal Uref.
Every switching change in the power switch 1 from the open to the closed state and back results in abruptly increasing current flows in the voltage transformer, which may be transmitted as perturbation or noise pulses to the output voltage Vout or to the power supply potentials Vcc and GND. This results in noise signals with the spectrum illustrated in FIG. 3 being superimposed, for example, on these potentials. The spectrum consists of discrete, equidistant lines at the frequency f=1/T of the reference signal Uref and integral multiples thereof. These noise signals can impair the functioning of circuits drawing energy from the same supply voltages as the DC voltage transformer or from its output voltage, as well as that of other nearby circuits.
What is needed is a DC voltage transformer in which the danger of disturbing nearby circuits by the switching of the transformer is considerably reduced.